BRUSSELS — Big banks found themselves under pressure in Europe's debt crisis Saturday, with finance chiefs pushing them to raise billions of euros in capital and accept huge losses on Greek bonds they hold.

The continent's biggest financial institutions were at the center of talks as leaders entered marathon negotiations in Brussels, at the end of which they have promised to present a comprehensive plan to take Europe out of its crippling debt crisis.

"Between now and Wednesday we have to find a solution, a structural solution, an ambitious solution and a definitive solution," French President Nicolas Sarkozy said as he arrived in Brussels. "There's no other choice."

In addition to new financing for Greece, leaders want to make the banking sector fit to sustain worsening market turmoil and turn their bailout fund into a strong safety net that will stop big economies like Italy and Spain from falling into the same debt trap that has already snapped Greece, Ireland and Portugal.

But before the final deadline on Wednesday, they have to overcome many obstacles.

On Saturday, the finance ministers of the 27-country European Union decided to force the bloc's biggest banks to substantially increase their capital buffers — an important move to ensure that they are strong enough to withstand the panic that a steep cut to Greece's debt could trigger on financial markets.

A European official said the new capital rules would force banks to raise just over euro100 billion ($140 billion), but finance ministers did not provide details on their decision. The official was speaking on condition of anonymity because it had been agreed to let leaders unveil the deal at their first summit Sunday.

"We have made real progress and have come to important decisions on strengthening European banks," George Osborne, the U.K.'s chancellor of the exchequer, said as he left Saturday's meeting.

The deal on banks was likely to be the only major breakthrough ready to announce on Sunday, leaving many important decisions and negotiations to be completed by Wednesday night.

On Friday, the first day of the marathon talks, the finance ministers of the 17 countries that use the euro — and which have found themselves at the center of the crisis because of the currency they share — agreed to demand Greece's private creditors take big losses on their bondholdings.

But they still have get the banks to come along and convince them that the cuts are the best way to ensure that Athens can eventually repay its remaining debts.

The picture in Greece, whose troubles kicked off the crisis almost two years ago, is bleaker than ever. A new report from Athens' international debt inspectors — the European Commission, the European Central Bank and the International Monetary Fund — proved that a preliminary deal for a second package of rescue loans reached in July is already obsolete.

That plan would have seen banks and other private investors take losses of some 21 percent on their Greek bond holdings, while the eurozone and the IMF were to provide an extra euro109 billion ($150 billion) in bailout loans.

But the report showed that in the past three months Greece's economic situation has deteriorated so dramatically that for the bank deal to remain in place, the official sector would have to provide some euro252 billion ($347 billion) in loans. Alternatively, to keep official loans at euro109 billion ($150 billion), banks would have to accept cuts of about 60 percent to the value of their Greek bonds.

"I believe we are now arriving at a more realistic view of the situation in Greece," said German Chancellor Angela Merkel, the country that has long been advocating a more radical solution to Athens' problems.

But Merkel and her eurozone counterpart were on for tough negotiations with the banks.

Charles Dallara, who has been representing private investors in the talks with the eurozone, said Saturday that negotiations that carried on sporadically throughout Saturday were making only slow progress.

"We're nowhere near a deal," he told The Associated Press in an interview.

Dallara, the managing director of the Institute of International Finance — the world's biggest bank lobbying group — said current plans to cut Greece's debt would leave the country as "a ward of Europe" for years.

He declined to say how much in losses banks would be willing to accept, saying only "we would be open to an approach that involves additional efforts from everyone."

The eurozone has been working hard to reach a voluntary agreement with banks, rather than forcing losses onto the lenders, because that could avoid triggering billions of euros on payout for bond insurance and could destabilize markets even further.

However, in recent weeks some officials have no longer insisted that the deal remain voluntary.

Agreement on arguably the most important measure in the crisis plan remained even more elusive Saturday: boosting the firepower of the currency union's euro440 billion ($600 billion) bailout.

Increasing the effectiveness of the fund — called the European Financial Stability Facility — is meant to help prevent larger economies like Italy and Spain from being dragged into the crisis. At the same time, the EFSF may be asked to help governments shore up their banks if they can't raise the necessary funds on financial markets.

But Germany and France still disagree over how to give the EFSF more firepower. France wants the fund to be allowed to tap the ECB's massive cash reserves — an option that Germany rejects. Weaker economies, meanwhile, are wary of signing up to the other two parts of the grand plan — bigger bank capital and cuts to Greece's debt — without assurance that sufficient buffers are in place.